Presence Of Nitrogen Vacancies Research Articles

A novel two-dimensional Janus TiSiGeN4 monolayer with N vacancies for efficient photocatalytic nitrogen reduction.

The photocatalytic nitrogen reduction reaction (pNRR) is a clean technology that converts H2O and N2 into NH3 under environmental conditions using inexhaustible sunlight. Herein, we designed a novel two-dimensional (2D) Janus TiSiGeN4 structure and evaluated the pNRR performance of the structure with the presence of nitrogen vacancies at different positions using density functional theory (DFT) calculations. The intrinsic dipoles in the Janus TiSiGeN4 structure generate a built-in electric field, which promotes the migration of photogenerated electrons and holes towards the (001) and (00-1) surfaces, respectively, to achieve efficient charge separation. For the pNRR, the Si atoms exposed after the formation of top N vacancies can realize the efficient activation of N2 through the "acceptance-donation" mechanism, while the presence of middle N vacancies not only suppresses the hydrogen evolution reaction, a competition reaction, but also lowers the reaction barrier for the protonation of N atoms. The limiting potential of TiSiGeN4 with the coexistence of both top and middle N vacancies (TiSiGeN4-VN-mt) is as low as -0.44 V. In addition, the introduction of N vacancies generates defect levels, narrowing the band gap and improving the light response. This work provides theoretical guidance for the design of efficient pNRR photocatalysts under mild conditions.

Synergistic enhancement of Ce–ZnO/g-C3N4 photocatalytic performance using N,O-bis-vacancy induction and S-scheme heterojunctions

The photocatalytic decontamination of organic pollutants is an environmentally friendly and efficient water treatment method. The introduction of vacancies, heterojunctions, and doping can enhance the performance of photocatalysts for such applications. In this study, N,O double-vacancy Ce–ZnO/g-C3N4 S-scheme heterojunction photocatalysts were prepared. It is found that the electron separation and transfer efficiencies are considerably improved by the presence of nitrogen vacancies in graphitic carbon nitride (g-C3N4) and oxygen vacancies in ZnO, in addition to the S-scheme heterojunction created between the two materials. In the degradation of methylene blue and ciprofloxacin, the degradation rates exhibited by Ce–ZnO/g-C3N4 are significantly higher than those of the g-C3N4/ZnO system. This remarkable photocatalytic performance of Ce–ZnO/g-C3N4 may result from three key factors: (i) Ce doping, which produces impurity energy levels; (ii) the generation of electron capture centres by the N and O vacancies; and (iii) the synergistic effect of the S-type heterojunction formed at the interface between the two materials. This study therefore provides new approaches for the use of doping, vacancies, and heterojunctions to induce the efficient degradation of organic pollutants.

Efficient photo-switchable activation of periodate by nitrogen-vacancy-rich carbon nitride for organic contaminant removal: Theoretical predictions and experimental validations

This study presents a simple N2 calcination method at various temperatures to achieve nitrogen-vacancy-rich g-C3N4 for periodate (IO4-) activation under visible light. The developed system demonstrated remarkable effectiveness in eliminating various organic pollutants. Especially, bisphenol A, the model pollutant, could be completely removed within 30 min, exhibiting an activity 9.37 times higher than that of the background activator. Theoretical calculations suggest that small active molecules, including contaminants and oxidants, showed a higher affinity for adsorption onto the g-C3N4 surface in the presence of nitrogen vacancies. This promoted sufficient electron supply and rapid electron transfer, enhancing IO4- activation during decontamination. Furthermore, we comprehensively examined the evolution of reactive species, such as IO3•, •O2-, 1O2,•OH, and h+, and explored the application potential of the catalytic system. This work provides new insights into the design of photo-self-cleaning IO4- activators and highlights this method's potential for application in other oxidation systems.

Thermally-induced co-existence of superparamagnetism and spin-glass like behavior in undoped amorphous AlN thin film

The co-existence of superparamagnetism (SPM) and spin-glass (SG) behavior in undoped amorphous AlN thin film is reported for the first time. The as-deposited AlN film exhibits only SPM behavior while post-deposition annealing at 850 °C for 4 h in a high vacuum of 2 × 10−6 mbar induces SG signature along with SPM. GIXRD measurement confirms the amorphous nature of AlN in as-deposited and annealed films. EDX, RRBS, and XPS measurements indicate the presence of nitrogen vacancies which are drastically enhanced after annealing. The magnetic hysteresis loop for annealed film elucidates the existence of soft and hard magnetic phases attributed to the presence of SPM and SG states, respectively. Temperature-dependent magnetization, magnetization relaxation, and Arrott plot confirm the presence of SG state. Based on the observations and the analysis, the thermally induced enhancement of nitrogen vacancies present in AlN are attributed to be responsible for the co-existence of the SPM and SG state.

Synthesis of Vitamin B3 through a Heterogeneous Photocatalytic Approach Using Metal-Free Carbon Nitride-Based Catalysts.

Vitamin B3 (nicotinic acid, VB3) was synthesized through the photocatalytic oxidation of 3-pyridinemethanol (3PM) under visible-light-emitting diode (LED) irradiation using metal-free graphitic carbon nitride (GCN) - based materials. A bulk (GCN) material was prepared by a simple thermal treatment using dicyandiamide as the precursor. A post-thermal treatment under static air and nitrogen flow was employed to obtain the GCN-T and GCN-T-N materials, respectively. The conditions adopted during the post-treatment revealed differences in the resulting materials’ morphological, electronic, and optical properties. The post-treated photocatalysts revealed an enhanced efficiency in the oxidation of 3PM into VB3, with the GCN-T-N photocatalyst being the best-performing material. The defective surface, reduced crystallinity, and superior photoabsorption of GCN-T-N account for this material’s improved performance in the production of VB3. Nevertheless, the presence of nitrogen vacancies in the carbon nitride structure and, consequently, the creation of mid-gap states also accounts to its highly oxidative ability. The immobilization of GCN-T-N in sodium alginate hydrogel was revealed as a promising strategy to produce VB3, avoiding the need for the photocatalyst separation step. Concerning the mechanism of synthesis of VB3 through the photocatalytic oxidation of 3PM, it was possible to identify the presence of 3-pyridinecarboxaldehyde (3PC) as the intermediary product.

Construction of nitrogen vacant g-C3N4 nanosheet supported Ag3PO4 nanoparticle Z-scheme photocatalyst for improved visible-light photocatalytic activity.

The superior photocatalytic activity of semiconductor-based photocatalytic materials has attracted great attention. In this work, a series of novel Ag3PO4/g-C3N4-x (APO/CNx) composites with the Z-scheme structure were fabricated through a facile precipitation method. B naphthol, a typical phenolic compound, was selected to evaluate the photocatalytic activity of all as-prepared photocatalysts. The obtained APO/CNx composites exhibited excellent photocatalytic activity for degradation of B naphthol under visible-light irradiation. Experimental results showed that the degradation rate toward B naphthol could reach to 90.5% for 180min, which was almost 3.66 times higher than pure g-C3N4, indicating that the presence of nitrogen vacancies and Z-scheme structure could efficiently improve the photocatalytic performance of pure g-C3N4. Furthermore, the results of trapping experiments and electron spin resonance (ESR) spectroscopy manifest that •O2- and •OH radicals were the predominant active substances for B naphthol degradation, and the possible mechanism of improved photocatalytic performance was elucidated. This work will provide an innovative perspective for constructing Z-scheme photocatalysts for the application of photocatalytic in the field of wastewater treatment.

Interfacial Charge Transfer between Silver Phosphate and W2N3 Induced by Nitrogen Vacancies Enhances Removal of β‐Lactam Antibiotics

AbstractConstructing heterojunctions has been demonstrated as an important approach to improve the catalytic performance of photocatalysts, but how to regulate the transfer and separation of photogenerated carriers at the interface is still a great challenge. Herein, W2N3‐NV, a two‐dimensional transition metal nitride containing nitrogen vacancies, is synthesized by a molten salt‐assisted atmosphere calcination method. A novel composite photocatalyst Ag3PO4@W2N3‐NV with good photocatalytic activity and photostability is prepared for the first time and applied to the efficient removal of β‐lactam antibiotics. The composite catalyst shows much superior photocatalytic degradation performance of penicillin and amoxicillin, and the apparent rate constant of which is 77.7 and 42.9 times than that of pure Ag3PO4, respectively. Experimental results and density functional theory calculations confirm that the presence of nitrogen vacancies can drive the formation of defects and dangling bonds on the W2N3‐NV surface, which make it easy to combine with Ag3PO4 and form new chemical bonds at the interface. The WO chemical bonds formed at the interface provide a fast transfer channel for the interfacial photogenerated charge, resulting in the boosted carriers transfer and separation ability of Ag3PO4@W2N3‐NV composite. This study provides a new strategy for the interface engineering of highly efficient heterogeneous photocatalysts.

Defective g-C3N4/covalent organic framework van der Waals heterojunction toward highly efficient S-scheme CO2 photoreduction

In this work, a novel van der Waals (vdW) heterojunction composite combining g-C3N4 with nitrogen vacancies and Tp-Tta COF manifests effective interface contact area and excellent photocatalytic CO2 reduction performance. First-principles density functional theory calculation and experimental results suggest that the presence of nitrogen vacancies in g-C3N4 can widen the Fermi level gap between C3N4 (NH) and Tp-Tta COF, promoting the recombination of invalid photogenerated carriers through S-scheme pathway. Benefitted from the accelerated transfer of photogenerated charges at the vdW heterostructure interface, the deactivation of oxygen vacancies in C3N4 (NH)/COF is prevented and much higher photocatalytic activity and stability are obtained. The efficient electron transfer and the affinity of Tp-Tta for CO2 are beneficial to the enhanced CO selectivity. This work provides insights for the design of S-scheme heterojunction photocatalyst for CO2 reduction.

Nitrogen Vacancy‐Induced Deposition of Pd Nanoparticles onto g‐C3N4 with Greatly Improved Photocatalytic Activity in H2 Evolution

C3N4 exhibits excellence in photocatalytic water splitting. However, some hindrances should be overcome before its wide application. Herein, nitrogen vacancies (NVs) are successfully introduced in C3N4. As‐fabricated NVs act as targets to induce the deposition of Pd nanoparticles (NPs). Photocatalytic activity in H2 evolution for C3N4 is improved from none to 10.12 μmol h−1 gcat−1 in the presence of NVs, and to 287.94 μmol h−1 gcat−1 with the modifications of both NVs and Pd NPs. The great improvement may be due to that: 1) the formation of NVs can drive up the Fermi level and optimize the band structure of C3N4; 2) the addition of the impure energy level of NV within the bandgap expands the utilization of the solar spectrum; 3) Pd NPs with the surface plasmonic resonance (SPR) effect are capable of absorbing more visible‐light photons; and 4) Pd acts as a reservoir of photogenerated charge carriers, suppressing its recombination. The mechanism of the enhancements is explored in detail and comprehensively discussed in this work.

Defect engineered mesoporous 2D graphitic carbon nitride nanosheet photocatalyst for rhodamine B degradation under LED light illumination

In this work, a nitrogen vacancy induced 2D mesoporous graphitic carbon nitride (g-C3N4) nanosheet photocatalyst was successfully synthesized through a simple two-step thermal treatment method. The morphology of the nanosheet photocatalyst and the presence of nitrogen vacancy was explored through a wide range of characterization techniques. The as-prepared photocatalyst possess an improved visible light absorption efficiency as confirmed from the UV–vis diffuse reflectance spectroscopy (DRS). Moreover, the improved charge carrier separation efficiency of the nitrogen vacant material was demonstrated from the photoluminescence spectrum. Most importantly, the photocatalyst exhibited an excellent photodegradation efficiency towards rhodamine B (RhB) dye under the illumination of an 18 W LED light. The vacancy induced nanosheets demonstrated a degradation co-efficient of 0.074 min−1 in RhB degradation, which is 9.25 fold higher than that of the bulk g-C3N4. The nanosheets further exhibited an enhanced degradation efficiency toward tetracycline antibiotics. Furthermore, the photocatalyst displayed outstanding stability even after 5 cycles. A plausible photocatalytic mechanism has also been explained based on the results obtained from the radical scavenging experiments. This study would provide insight into the defect induction mechanism into the 2D g-C3N4 nanosheet and expected to help in rationally designing vacancy induced materials with cost-effective application in various environmental fields.

Synthesis of porous few-layer carbon nitride with excellent photocatalytic nitrogen fixation

The porous few-layer g-C3N4 (PFL-g-C3N4) is prepared by a simple molecular self-assembly method. Compared with pure g-C3N4, the as-prepared PFL-g-C3N4 is ultrathin, the surface is rich in pores, and the photocatalytic nitrogen fixation activity is greatly increased to 8.20 mM h−1 gCat−1. Few-layer and ultrathin nature of PFL-g-C3N4 can provide a larger specific surface area, expose more active sites, and reduce the diffusion path of charges and protons from the inside to the surface. In addition, the porous structure can narrow the band gap, thereby increasing the light absorption range and enhancing the light absorption capability. Meanwhile, the presence of nitrogen vacancies causes PFL-g-C3N4 to move to a more negative conductive band value. More importantly, the isotopic experiments using 15N2 as nitrogen source confirm the ammonia production originating from N2 rather than the decomposition of g-C3N4. Therefore, PFL-g-C3N4 can greatly improve the efficiency of visible light photocatalytic nitrogen fixation.

Graphitic carbon nitride tetragonal hollow prism with enhanced photocatalytic hydrogen evolution

Graphitic carbon nitride tetragonal hollow prism (GCN-THP) with nitrogen vacancies was prepared by a simple two-step calcination method. Based on the characterizations of the as-prepared GCN-THP and the intermediate precursor, a possible mechanism was proposed for the formation of GCN-THP. The as-prepared GCN-THP exhibits superior activity and excellent stability during photocatalytic hydrogen evolution under visible light irradiation. The photocatalytic hydrogen evolution rate of GCN-THP was measured to be 1990 μmol g−1 h−1, which is 6.2 times as that of GCN. The enhanced photocatalytic activity could be attributed to unique 1D tetragonal hollow prism morphology and the presence of nitrogen vacancies in the as-prepared GCN-THP, which could increase the surface area, expand the visible light absorption, and promote the charge separation during photocatalytic hydrogen evolution. Our work could provide a new route to synthesize highly efficient photocatalysts with 1D hollow structures.

The critical role of N-vacancy on chemical composition fluctuations and degradation of InAlN layer

Due to its intrinsic properties and the possible lattice match to GaN, InAlN is expected to allow the fabrication of optimal high electron mobility transistors for high power and high frequency applications. However, the crystal quality of InAlN nearly lattice-matched to GaN degrades when the layer thickness is increased, and this is a strong limitation for the fabrication of devices in which thick barriers need to be used. In this work, we have carried out a detailed theoretical investigation of the behavior of indium atoms in the alloy. It is clearly shown that in the presence of nitrogen vacancies, which are common defects in these materials, indium nitride clusters will present excess formation energy up to diameters around 1.4 nm. In parallel, Z-contrast TEM observations close to the InAlN/GaN interface show that 2–5 nm size indium rich areas form and are systematically connected to the vertical degradation channels. This is at variance with published results, which concluded that the observed degradation was exclusively either due to the underlying threading dislocations or due to a characteristic three-dimensional growth mode.

Electronic and Magnetic Properties of Mn-doped and (Mn,C)-codoped w-AlN with the Presence of N Vacancy

In this paper, we discover the structural, electronic, and magnetic properties of Mn-doped (6.25 and 12.5%) and Mn-C-codoped wurtzite AlN with the presence of nitrogen vacancy. These properties have been investigated using density functional theory implemented in Quantum ESPRESSO package. AlN doped and codoped were optimized by using Broyden-Fletcher-Goldfarb-Shanno (BFGS) approximation. The electronic and magnetic properties are determined by the total and partial density of states. The presence of impurities and nitrogen vacancy induces magnetic properties, indicating that these materials are good candidates for electromagnetic and spintronics applications. Our results are in good agreement with experimental and other theoretical studies. We can conclude from this study that this material can be used as diluted magnetic semiconductors.

Enhanced thermal stability and mechanical properties of nitrogen deficient titanium aluminum nitride (Ti0.54Al0.46Ny) thin films by tuning the applied negative bias voltage

Aspects on the phase stability and mechanical properties of nitrogen deficient (Ti0.54Al0.46)Ny alloys were investigated. Solid solution alloys of (Ti,Al)N were grown by cathodic arc deposition. The kinetic energy of the impinging ions was altered by varying the substrate bias voltage from −30 V to −80 V. Films deposited with a high bias value of −80 V showed larger lattice parameter, finer columnar structure, and higher compressive residual stress resulting in higher hardness than films biased at −30 V when comparing their as-deposited states. At elevated temperatures, the presence of nitrogen vacancies and point defects (anti-sites and self-interstitials generated by the ion-bombardment during coating deposition) in (Ti0.54Al0.46)N0.87 influence the driving force for phase separation. Highly biased nitrogen deficient films have point defects with higher stability during annealing, which cause a delay of the release of the stored lattice strain energy and then accelerates the decomposition tendencies to thermodynamically stable c-TiN and w-AlN. Low biased nitrogen deficient films have retarded phase transformation to w-AlN, which results in the prolongment of age hardening effect up to 1100 °C, i.e., the highest reported temperature for Ti-Al-N material system. Our study points out the role of vacancies and point defects in engineering thin films with enhanced thermal stability and mechanical properties for high temperature hard coating applications.

Effect of the growth temperature and nitrogen precursor on the structural and electrical transport properties of SmN thin films

ABSTRACTWe report on the structural and electrical properties of epitaxial SmN thin films grown by molecular beam epitaxy. The effect of the growth temperature and nitrogen precursor, either pure molecular N2 or NH3 was investigated. The structural quality of SmN was assessed by X-ray diffraction and the epitaxial growth character is observed over the entire range of growth temperatures, from 300°C to 800°C, with both nitrogen precursors. The highest quality films are produced at a growth temperature of about 430°C by using N2 as a nitrogen precursor. Hall Effect and resistivity measurements establish that SmN films are heavily n-type doped semiconductors, suggesting the presence of nitrogen vacancies, a recurring phenomenon in rare earth nitride compounds.

Electronic structure and metallization effects at threading dislocation cores in GaN

We have carried out a complete and consistent study on core configurations of the three types of threading dislocations in wurtzite GaN. Their atomic and local electronic structures were analyzed and systematically compared, at the level of Density Functional Theory. Screw and mixed threading dislocations were found to introduce both deep and shallow gap states, while most of core configurations of the edge dislocation introduce only shallow states. We demonstrated that the existence of an extended one-dimensional metallization, associated with unoccupied gap states, is a specific feature of threading screw dislocations. The extended metallization along with the high dispersion of the energy gap states are at the origin of the experimentally observed high electric conduction along threading screw dislocations in GaN. The presence of nitrogen vacancies in the core of screw dislocations is predicted to enhance their electric conduction.

Characterisation of defects in rare earth implanted GaN by deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) was used to investigate the electrical properties of GaN implanted with the rare earth (RE) ions erbium and thulium. The GaN layers have been grown by metal-organic chemical vapor deposition (MOCVD) onto (0001) sapphire substrates. We used the channeled implantation geometry to implant a dose of 5×10 14 RE cm −2 with an energy of 150 keV. For each species, two different annealing procedures were used in a nitrogen atmosphere for 120 s. Indeed, the annealing temperature plays an important role in the lattice recovery, even if RE-related defects remain present. After annealing at 1000 ∘C, the appearance of two new peaks, for both studied RE ions, is associated with the lattice damage induced by the implantation, such as the presence of nitrogen vacancies. After annealing at 1100 ∘C, the recovery of the lattice is observed while a hole trap appears for both implanted RE ions with corresponding energy values E v + 0.61 eV and E v + 1.59 eV, in the case of Er and Tm, respectively.

Vacancy-induced modifications to the local densities of states of VNx

The valence states of vanadium nitride have been studied by X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) as a function of the nitrogen vacancy concentration. Vacancy induced states are observed in the V K emission band (and in the XPS valence band spectrum) ~2eV below the Fermi energy suggesting that the V p states are strongly affected by the presence of nitrogen vacancies. This is confirmed by simulations of XES and XPS spectra from Korringa-Kohn-Rostoker coherent potential approximation densities of states.

X-ray-emission-spectroscopy study of vacancy-induced electronic states in substoichiometric TiNx.

The Ti K emission band from TiN and substoichiometric ${\mathrm{TiN}}_{\mathrm{x}}$ (0.35\ensuremath{\le}xl1) has been investigated. The Ti p--like states are very strongly hybridized with the N 2p states located \ensuremath{\sim}6 eV below the Fermi level (${E}_{F}$) in TiN. In the presence of nitrogen vacancies, p-like nonbonding states are detected \ensuremath{\sim}2 eV below ${E}_{F}$. As x decreases, the intensity of the nonbonding structure increases, that of the \ensuremath{\sim}6 eV peak decreases, and the valence-band states move closer to ${E}_{F}$ reflecting the decrease in cohesion. Theoretical x-ray spectra have been obtained with use of the Korringa-Kohn-Rostoker coherent-potential-approximation densities of states and appropriate transition-matrix elements. The theory is shown to account for all the trends observed experimentally.